High-strength rolled steel sheets

ABSTRACT

A HIGHT-STRENGTH ROLLED STEEL SHEET OF CHEMICAL COMPOSITION COMPRISING; C: 0.02-010%, SI$0.7%, MN: 0.01-0.70%, P: 0.05-0.25%, AI: 0.005-0.100% WITH THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES.

All@ 6, 1974 HlRosHl TAKEcHl UAL 3,827,924

HIGH-STRENGTH ROLLED STEEL SHEETS Filed May 19, 1972 3 Shoots-Shut 1 Tensile StrenQtMKQ/mmZ) Allg. 6, 1974 HiRosHrTAKEcI-ii ETAL 3,827,924

HIGH-STRENGTH ROLLED STEEL. SHEETS Filed May 19, 1972 F Q 3 s shntsmet 2 1.8 O O o r value 1.5 A E O- l-nve-ntve-Stdeel 3.1- OO O O A vA Limit Prim ary Work {imm/ing Rute 9 o o A A Limit Drawin Rate `-T.v Which does ngt 2B A O inventive Steel 1| Cause Embrttiement 2.7 @Comparative SieelJl Cracking in Impact Secondary Working 2.6 Test 2.5 A A A ug. 6, 1974 HIROSHITAKECHI ETAL 3,327,924

HIGH-STRENGTH nomma STEEL' SHEETS Filed May 19, 1972 Fl G .3 Shotsheot I5 100 h Test Conditions C Relative Humidty90l 30days V**' R st Fo 1.o 70 l O ICnventive Steel i u rma n A om a t' Per UnitAreu(/.) 60- A L p G 'J' A A 2OFOOOOOO OAA A AA AAAA 10. O A A 0 icni: il-c'ai'i j kLMNeosruv'i/X lnventive Comparative f Steel Steel 220 Occurrence of o A l llO 200 Phosphate Nuclei Phosphate Crystal Grain Diameter o A 90 A 15o-o 000A A AA AAA-8o Occurrence of 0 A Phosphate 4 140 O O o A A 70 Nuclei (X10 /cm2 A 20 so Emir??- rys a ram 00 A 50 Diemeterl/l) A 60- A A 30 1.o- A A A 2o 2O .....0AAAAA A AAAA 1O A 0 ACFGH i J KLi/iNoeoRsTuvwx` O United States Patent lce 3,827,924 Patented Aug. 6, 1974 Japan Filed May 19, 1972, Ser. No. 255,019 Claims priority, application Japan, May 21, 1971, 46/34,625; Feb. 12, 1972, 47/ 14,321 Int. Cl. C22c 29/00 U.S. Cl. 148-36 2 Claims ABSTRACT F THE DISCLOSURE A high-strength rolled steel sheet of Chemical composition comprising;

C: 0.02-0.l0%, Si0.7%, M11: Q01-0.70%, P: 0.05-0.25%, Al: 0.005-0.l00%

with the balance being iron and unavoidable impurities.

The present invention relates to a high-strength rolled steel sheet having good drawability, impact-resistance,

1. Excellent drawability as well as high-strength as compared with that of ordinary cold rolled steel sheets for general use, and little susceptibility to embrittlement.

2. Excellent corrosion-resistance and coatability as automobile trims.

Among the factors indicating elongation and ductility of steel sheets, the workhardening index n is used. This n value is a value given by the ratio of the tensile strength (or yield point) and strain e, namely dlog "dlog e A large n value shows better elongation and ductility of the steel sheet, and this value lowers as the tensile strength of the steel sheet increases.

Further, steel sheets have the tendency of secondary work cracking under which embrittlement or pseudo-embrittlement destruction occurs is due to loads imposed on work-pieces by deep-drawing. In general, the secondary work test is done to determine if destruction (or cracking) is results on the side wall portion etc. of a cup test sample by drawing a steel sheet having an 80-160 mm. diameter with an appropriate ratio of reduction area (ratio of primary drawing). The thus obtained cup test sample is given loads continuously up to a maximum load of 3000 ton in a vessel containing ice and water. Generally, the larger limit ratio of primary drawing which does not cause cracking indicates a. better secondary workability.

Meanwhile, the drawability of cold rolled steel sheets may be determined by the value which is the average plastic strain ratio of a plastic strain ratio value) expressed by the ratio of sheet Width strain and sheet thickness strain, and thus the drawability may be expressed by the following formula 1`=(1ol2"45+"9o)14 r0=r value in the rolling direction r45=r value in a direction at 45 to the rolling direction rgozr value in a direction at to the rolling direction.

In general, a large value indicates better drawability, and ordinary Al-killed steels show an value of about 1.6, and rimmed steels show an value of about 1.3.

In the case of cold rolled steel sheets, many experiments and studies have been made with respect to the ad dition of special elements and thermal production methods chiey for the purpose of improving drawability alone instead of improving both strength and drawability. However, even with a cold rolled steel sheet having both high strength and high value, its application would be limited unless the products obtained by drawing are not susceptible to embrittlement cracking. The embrittlement cracking usually takes place in drawn products having a larger ratio of reduction of area, and as for the method for determining the cracking tendency, the secondary work test as mentioned above is adopted.

Further, in the case of cold rolled steel sheets, better surface properties, such as, rust-proofness and coatability as required by automobile applications and house surfacing materials and building construction surface materials have been increasingly in demand.

Therefore, the object of the present invention is to pro- Vide a high-strength cold rolled steel sheet which overcomes the above defects and satises the above demands and the present invention is based on the discovery that high P content, Si-P cold rolled steel sheets containing B, Al and Si or containing lowered oxygen content shows excellent material qualities and surface properties.

The basic composition of the present inventive steel is:

C: 0.02-0.l0%, Si0.7, Mn: U01-0.70%, P: 0.05-0.25%, Al: 0.0050.l0%

with the balance being iron and unavoidable impurities and this basic composition is modified by the addition of 0.002-0.0l% of boron or by reducing the oxygen content to 0.015% or less.

Typical embodiments of the present inventive steel comprises:

C: 0.020.10%, Si0.7%, Mn: 0.0l0.60%, P: 0,050.25%, Al: 0.005-0.l00%, B: 0.002-0.0l0%

with the balance being iron and unavoidable impurities,

with the balance being iron and unavoidable impurities.

Reasons for the limitation of each element in the present invention are as follows:

Carbon is an element eective for increasing the strength of steel sheets, but it is rather used as an element for improving impact secondary workability. Less than 0.02% of carbon is not effective, while excessive carbon content forms large cementite and lowers the ductility impact secondary workability and the value. Therefore, the upper limit of the carbon content is set at 0.100%.

Silicon is eifective for increasing strength, but in the present invention it contributes to prevent the lowering of the impact secondary workability due to phosphorus and more than 0.11% of Si is required.

But silicon contents of more than 0.69% tend to deteriorate the improved rust-proofness and coatability contributed by the phosphorus content.

Manganese is effective for increasing the strength and is contained as an important component for high tensile strength steel sheets. lIn the present invention, more than 0.01% of Mn is required for increasing the strength and preventing hot embrittlement during hot rolling. But manganese contents of more than 0.70% will lower the drawability and rust-proofness improved by phosphorus content.

`More than 0.05% of phosphorus is required for improving the strength, drawability as Well as the coatability and rust-proofness, but more than 0.16% of phosphorus will deteriorate the impact secondary workability. Boron is added to prevent the deterioration of impact secondary workability caused by addition of phosphorus more than 0.16% and thus phosphorus can be safely added to up to 0.25%, but a preferred range of phosphorus content is E-0.15%. More than about 0.002% of boron is necessary for the above purpose but excessive boron content will deteriorate ductility, press formability and cause surface cracking of hot rolled slabs. Thus, the boron content is limited to 0.002-0.010%.

In general, aluminum acts as a deoxidizing agent and the present inventors have found that aluminum is effective for preventing deterioration of the impact secondary workability, and at least 0.01% of aluminum in the form of soluble aluminum is required for the above purpose in the case where no boron is added. On the other hand, when boron is added, aluminum is an amount more than 0.005% enhances the effect of boron by being combined with oxygen and nitrogen in the metal to improve the secondary workability. But an excessive aluminum content increases the alumina content to deteriorate the surface properties with only little improvement in its desirable effect. Thus the upper limit of aluminium is limited to 0.10% in the form of soluble aluminum.

Silicon is an effective element for deoxidation and improvement of strength, and is necessary in the present invention for improvement of the secondary workability thus increasing the effect of boron addition by combination with oxygen just as aluminium. But more than 0.7% of silicon will lower ductility and should be avoided because it damages the press formability.

Oxygen is present in the steel as non-metallic inclusions and in a solid solution state in a very small amount. When oxygen is present in an amount of more than 0.015%, it has been found, that impact secondary workability is remarkably deteriorated and oxidizes useful elements, such as, silicon and phosphorus and lower their effectiveness. Therefore, the upper limit of the oxygen content is set at 0.015% and the oxygen content should be as low as possible.

Unavoidable impurities, such as, sulfur which bars the object of the present invention and often causes crackings during the production of cold rolled steel sheets, should be kept as low as possible.

The high strength cold rolled steel sheets of the present invention are produced by making steel of the above composition in a converter or an electric furnace, making ingots from the steel, breaking the ingots into slabs, or continuous-casting the steel into slabs, hot rolling the slabs, acid pickling and cold rolling the hot rolled plates with a reduction rate of more than 30%, box annealing 0r continuously annealing the sheets at a temperature between 550 C.-900 C., and then skin-pass rolling the sheets with a reduction rate between 0.2-3.0%.

The present invention will be better understood from the following examples.

Example 1 Steel having a chemical composition shown in Table 1 was prepared in a 1-ton electric furnace. Thus obtained steel ingot was soaked for one hour at 1250o C., hot rolled at 900 C. at a finishing temperature of 900 C. to obtain a hot rolled plate of 2.7 mm. thickness, and the hot rolled plate was cold rolled at a reduction rate of 70%, subjected to a batch type recrystallization annealing in which the sheet was kept at 710 C. for 4 hours and then furnace cooled, and then skin-pass rolling at a reduction rate of 1.2% to obtain cold rolled steel sheets. J IS No. 5 tensile test samples were prepared from the above obtained cold rolled sheets and subjected to an ordinary tensile test by a tensile test machine of instron type to determine yield point, tensile and n values. The test results are shown in Table l. The 11 value was obtained by the following formula;

in which :11 value in the rolling direction of the sheet 1145=11 value in a direction at 45 to the rolling direction 1190=11 value in a direction at 90 to the rolling direction.

The yield point and tensile strength values were obtained by the same method.

Example `2 Table 3 shows chemical compositions of cold rolled steel sheets of 0.8 mm. thickness produced by making ingots from molten steel prepared in a convertor, breaking down the ingots and hot rolling the slabs into hot rolled steel plates of 2.7 mm. thickness and cold rolling the hot rolled plates.

The cold rolled steel sheets were further subjected to recrystallizat'ion annealing at 710 C. for 4 hours, and to skin-pass rolling at a reduction rate of 1.2% to obtain the high-strength cold rolled steel sheets t(A-H) of the present invention. The steel sheets (I-X) are shown in Table 3 for comparison.

Tensile strength values of these steel sheets are shown in Table 3 and 11 values are shown in FIG. 3 and the limit drawing ratio values obtained by the impact secondary wortking test `(these values were obtained for samples prepared by making the outer circumference of steel blanks before primary drawing) are shown in IFIG. 4, rustproofness and coatability are shown in FIG. 5 and IFIG. 6, respectively. The coatability was determined by paint adhesion test. The paint adhesion test is done by subjecting the steel sheet to phosphate treatment and determining the diameter phosphate crystal grains and the number of phosphate nuclei per unit area v(l cm?) of the sheet. A smaller diameter of phosphate crystal grains, and a larger number of phosphate nuclei indicate better coatability.

The present steel sheets show better properties than those of the comparative steel sheets, and particularly in respect of the results of the pa-int adhesion test shown in FIG. 6, the present inventive steel sheets show a phosphate crystal grain diameter of about 114511 and phosphate nuclei of about -170 104/cm.2 as compared with a phosphate crystal gram diameter of about 15--171/I and phosphate nuclei of about 7090 104/cm.z of ordinary rimmed soft steels, thus indicating the excellent coatability of the present inventive steel sheets.

It should be noted in this point that the present inventive steel sheet shows an excellent value in respect of each of drawability, impact secondary workability rust-proofness and coatability (paint adhesion) as shown lin FIG. 3 to IFIG. 6. In case of the comparative steel sheets, one or more of these properties are remarkably poor and thus very unsatisfactory lfor the applications aimed by the present invention, and are greatly limited their use as highstrength cold rolled steel sheet.

TABLE l-Continued TABLE 1 (Samples AB-AIO, A14-A17: Inventive steel sheets, Samples A1-A9,

Mechanical properties A11-A13, A18, B1-B12: Comparative steel sheets) Bamples 87 26 44 12 1 www AWM 22 22 22 ww 0.0. 0.0. 0.0 0.0. 0.0. 0.0. 12 16 09 13 82 27 7.6 6.5. .7. 7.6. W 44 33 M3 @ma 33 3% 28 49 60 19 49 81 m4 .4 2.0m 4 0. 8 4 L 33 2 32 33 22 3W 0 1 6 7 8 9 1 1 B B B B B B 0 5 1 1 434 6745873446746 3 00 0 00 00000 oowmumwmmoomwooooo O.O.0 U. U.O.O.O.00.0.0.0.0.0.00.0. 11111184.28 757251 0 0 wwwmmowwmmwmmno 0.nw0.0.00.00.00.0.0.0.0.0.0.00. 2424823222 485143 00 17 WW.. 361 0 OOMWMMWWOOMO @000Mo 000.00000000000000@ 28696 464267 2 53 43 0 100 0% m0 m OOOWmOnWOOOw 0m 0.0.00.0.0.0..0000.0.0.0.&0.0.0. 86445681000036043664 1 21 1 96 1 momamonmncmani 0.0.00.0.nv.0.0.0n0.0.0.0.0.0.0.0 82644523644905469686 01u12 22 1 11 11 MOOOMMWOWOMWOOWOO 4, A 1 en A 22 s 0.0. B .M ma a S h s s t @um h e s .t s 2.8M du. W 0 4 .mn s a 2 i e a E .wm L t n 0 B 6C A V n om T I 1 w1. ma. 2 33 AB M1... A u yA 0 y 19 AA u or7 AA S e 1 D. 2 m 1 S B o 2 2 344333799M43 000 00 wwmmwn 0.0.0.0.0.0.0.Q0.0.0.0 111111111111 00000 0.0.0.nm0.0.0.0.0.0m0.0 000 4430 0.0.0.0.0.0.0. L10.00 83 Bran/32H14 0 32 mmmmwomooo Qnmnv.0.0.0.0.0.0.0.0.0 346482819648 11 mmmwmm 0.0.0..0.1Q0.0.0.0.0. 289468439863 0 2 mmmmmmOmmO 0.0.0.0.0.0..0.0.0.0.0. 694186426 0.L20.0.0.0..0.0.0.0. 1 1 11 mmmmmmOmOmOO Chemical Compositions (weight percent) Sam- Mechanical properties ples n Value Yield point Tensile strength (kg/mm?) (kg/mm!) Samples 000 o 0 0 0.000.000.0000. 9 88757259. mwwmwmm 00.00.00.00.00. 22 24485143 O0 36 mwwoooomm 0.00.0.0.0.0.0.0.0.0. 6 267823538 1 00 ommooww 0.000.000.0000. 18836043664 2 2 6 omwnanzann 0.0.0.0.0.0.00.0.0.0. 36485469686 12 moommmmm 0.000.000.0000. 96 2415 000000.00000 6 wwmwnrnnm 0.000.000.0000. 63944682593 4 25 2 6 OMOOMOMOWWW 0.0.0.0.0. 0.0Q0.0.0 2 s ewurnnwnn. AAAAAAAAAA 5 0 3 4. 98 27 27 9G 68 11 34 45 MMM 22 22 HH 22 00. 0.0. 0.0. 0.0. 0.0 32 26 69 22 26 90m .3 .0. UM 44 M4 M M A. 84 16 91 12 98 0.5. 6.2 2.& a 4 5 6 7 A A A A A Secondary Workability XXXXXXXXXOXXXOOOOX XXXXXXXOOOXXXOOOOX $OOOOOIOOOOOQO 16 30 39 58 97 28 8l 80 01 23 2 34 57 01 www Q/M 22 22 m2 22 22 22 0.0 00. 0.0 0.0. 0.0. 00 0.0 0.0 om 5.4M .6. .2. 32. 9 M5 MP. 4 44 4 4 44 4 12 26 46 19 68 15 23 79 um-0. .L 0 4 .0.1m '0.1L 0.5m WW Mod 33 3 33 33 33 43 u n w u n @lo M .6..v 11. A A A A A A A A REMARKS: In the column of Secondary Workability" pnmary draw. ing ratios are indicated. This means primary working tests were done by deep-drawing at the respective drawing ratios. Then the drawn cup test pieces were imposed with a static compression load up to 3 tons to determine if embrittlement or pseudo-embrittlement cracking is caused Qui 22 on the side Wall of the cup test pieces. O and X show results obtained after maintaining the test pieces at 710 C. for 4 hours and then furnace cooling, and O indicates no cracking is caused and X indicates crackin TABLE 3 Chemical Compositions (weight percent) Tensile strength Sol. (kg./

Samples C Si P Mn S Al O mm!) Invontivo 0. 42 0. 151 0.37 0. 010 0. 016 0. 011 50. 6 Comparative steels:

What 'is claimed is:

1. A high-strength rolled steel sheet consisting essentially of C: 0.020.10%; Si0.7%, Mn: G01-0.60%,

P: ODS-0.25%, Al: 0.005-0.10%,

B: 0.002-0.010%, and 0.005 to 0.008% N with the balance being iron and unavoidable impurities,

and having a yield point in the range from 29.7 to 35.9 kg./mm.2 and an n value from 0.210 to 0.267.

2. A highstrength rolled steel sheet consisting essentially of C: 0.025-0.100%, Si: UJI-0.69%,

Mn: 0.01-0.70%, P: 0.06-0.16%,

Al: 0.01-0.l0%, and Oxygen0.0l5%, with the balance being iron and unavoidable impurities, and having a yield point in the range from 29.7 t0 35 .9 kg./ mm.2 and an n value from 0.2J10 to 0.267.

References Cited UNITED STATES PATENTS v lMatsukura et al 1148-412 3,496,032 2/1970 Shimizu (II) et al. 148-12 '3,188,250 '6/-1965 Holbein et al '148-120 3,668,016 6/1-972 Shimizu l'(11) et al. 14S-36 2,772,154 11/1956 Morgan et al. 75-123 B 90 3,347,718 110/ 1967 Carpenter et al. 148-111 8,709,744 1/ 1973 Goodman et al. 148-36 3,244,565 4/1966 Mayer et al 048-1211 3,404,047 10/1968 Goodman et al. '14S-12:1 l3,537,913 1'1/ 1970 Klisowski 148-36 25 WALTER R. SATTERPIELD, Primary Examiner Urs. c1. x.R. 

